The invention relates to a process for the purification of waste waters accumulating from pulp production, particularly from chlorine bleaching of pulp.
It is known that great volumes of water are used in pulp production. As a result of this, large volumes of waste water also accumulate in the course of the processing, which cannot be discharged into the environment without being purified, because they contain various solid, colloidal or dissolved products. Because they contain numerous environmental pollutant materials, the legal provisions relative to waste water purification have recently become more severe, and adherence to these rules introduces new, difficult, technical and economical problems, particularly to the pulp, paper and cardboard manufacturer.
The waste water accumulating from pulp and paper production contains large quantities of organic products dissolved from the raw wood product, particularly lignin and hemicelluloses, which are present for the most part as colloidal or true solutions in the waste water. One particular problem is produced by waste waters coming from the pulp bleaching unit, because they contain a considerable quantity of chemicals used in bleaching or their reaction products, which, on account of their toxicity, have an extraordinary environment polluting effect. The content of substances with mutagenous properties in the waste waters of pulp chlorination is particularly critical, as has been established by L. Stockman, L. Stromberg, F. DeSousa, Cellulose Chem. Technol. 14 (1980), pages 517 to 526. According to this, compositions such as 1,3-dichloroacetone, monochloroacetaldehyde, trichloroethylene, 2-(3-)-chloropropanol, chloroform and carbon tetrachloride, which are found in such waste waters, act as active mutagenous substances. For example, 1 to 5 mg/l of chloroform was found in a sample of waste water, which corresponds to a quantity of 40 to 200 g of chloroform per ton of pulp. Although some of these compositions are relatively unstable, there are still others which remain relatively stable over a long period of time under normal conditions. A rapid and effective method of making these compositions ineffective within the framework of the purification of such waste water would thus be very desirable.
The processes used up to the present time for purification or elimination of the large volumes of waste water accumulating from pulp production and particularly from the chlorine bleaching of the pulp are mostly rather time-consuming and costly and still do not provide the desired satisfactory results in terms of waste waters which pollute the environment as little as possible.
In the past, the waste water accumulating from pulp production and chlorine bleaching of pulp were frequently boiled down and the residues subsequently burned. Disregarding the high energy cost accruing with the boiling-down, large amounts of halogen are released from the bleaching waste waters with burning of the residues, which leads to extensive pollution of the environment, and therefore is very unsatisfactory.
Other solutions of the waste water problems which arise have been suggested, e.g. superfiltration or adsorption methods. However, it has been shown that these known methods are generally not suitable for continuous purification to the required degree of the waste water charged with a large load of dirt, so that it can without hesitation be fed back into the water system. It has particularly been shown that the waste waters which are so pre-treated cannot be fed into a biological purification, since they still contain chemicals from the bleaching which damage the microorganisms in a biologically activated sludge installation, and thus would greatly impair the process. That is why such processes have not been used extensively in the pulp, paper and cardboard industries.